1. Field of the Invention
The present invention relates to a method for producing a new type of connecting piece of metal or another electrically conducting material, preferably a cable shoe, that is to be joined to another object of metal or another electrically conducting material by means of brazing and in which heat is produced by an electric arc. The shape and construction of the electrically conducting connecting piece prevent the formation of structural changes (martensite formation) underneath the brazed joint. The invention also relates to such a connecting piece, preferably a cable shoe, of metal or another electrically conducting material.
2. Description of Related Art
Developments in railway traffic involve ever higher speeds and heavier axle loads. This in turn places increasing demands on the strength of railway track and its ability to withstand wear and accordingly rails are manufactured from higher-alloyed steel in order to meet these more stringent requirements. The material from which rails are manufactured is sensitive to thermal influences that can cause structural changes known as martensite formation (hardening effect).
Martensite formation may lead to crack formation in the rail material and due to the higher loads the rail may fracture, with catastrophic consequences for railway traffic. Consequently it is very important to braze signal and other wiring and cabling firmly to the rail by using a method that does not cause martensite formation in the rail.
Up to now it has only been possible to minimise martensite formation or structural changes by means of a pin brazing method, which is described in Swedish patent 9003708-6 (469 319). Hitherto it has not been possible by any pin brazing method to completely eliminate martensite formation in electrical contact connections that are intended to join two or more objects by means of an electrical connection.
The most serious problem in current pin brazing methods employed on railway track is the large amount of heat that is produced underneath the brazing joint, which is caused by the electric arc that is generated in the brazing process and produces a deleterious structural change or martensite formation. The problem is to a large extent due to the pin brazing method that is currently employed. The flux material and silver solder that are required in the brazing are supplied via a brazing pin that is secured in the brazing gun and that at the same time forms an electrode.
In order to produce a connection between metal surfaces on rails/pipework and cable shoes, the current procedure is to provide a cable shoe with an opening so that flux material and brazing material can pass from the brazing pin through the cable shoe and then firmly braze the cable shoe to the flat metal surface. At the start of the brazing process the electric arc acts directly on the rail and produces locally a very high temperature which is then transmitted indirectly via the braze melt and generates high temperatures directly in the rail, which have a deleterious effect on the latter. There is also the risk of an alloying of electrode material in the brazing material when using conventional pin brazing systems, which has adverse effects on both the brazing and workpiece. The final stage in the currently employed pin brazing involves pressing the brazing pin downwardly into the brazing and then breaking off the pin, which has a negative effect on the brazed joint itself.
Furthermore, a disadvantage of current brazing processes is that both the flux material and brazing material are used in order to secure the electrically conducting connecting piece, preferably a cable shoe. Since the brazing process takes place over a short period, a large amount of heat is required which is transmitted from the electric arc via the brazing material through the cable shoe to the workpiece or rail. The problem has been that a satisfactorily secure brazing has to be produced at the same time without causing any structural changes underneath the brazing joint.
At the present time in France, Italy, Switzerland, Spain and Germany it is prohibited to use pin brazing on railway track on account of martensite formation.
Certain other countries where pin brazing on railway track is currently permitted are in the process of changing their requirements and consequently existing pin brazing methods will no longer be allowed to be used in the future.
The present invention relates to a new type of electrically conducting connecting piece of metal or another electrically conducting material, preferably a cable shoe, which involves a new method of temperature-controlled brazing in which the problem of martensite formation that affects other methods is solved.
The invention also relates to a method for implementing this new type of connecting piece of metal or another electrically conducting material, preferably a cable shoe.
An object of the present invention is to provide a connecting piece of electrically conducting material, preferably a cable shoe, that has a continuous compact flat end of electrically conducting material on which the electric arc acts during the brazing process in such a way that the arc does not come into direct contact with the workpiece, for example a rail, during the brazing process, and that the connecting piece together with the compact plate forms part of the temperature-controlled brazing process in which a brazing is obtained that is completely free of martensite underneath the brazed joint.
Another object of the present invention is to be able during the fabrication to press a brazing material in the form of a clip on the connecting piece and also place a flux material between the connecting piece and the brazing clip during fabrication. This saves a workstage and solves the problem of brazing per se in the workplace.
Moreover, an advantage is that brazing material does not need to be supplied from other units to the brazing joint per se in order firmly to braze the connecting piece, nor do the surfaces between the connecting piece and brazing material need to be cleaned in a separate operation.
A significant advantage of pressing a brazing material clip firmly on the underneath side during the fabrication of the connecting piece is that a uniform thickness is obtained on the brazing clip. This is shaped so that it is larger than the connecting piece per se, preferably a cable shoe, and projects beyond the edges of the latter. This forms a more solid and more complete assembly of the cable shoe and workpiece, and also prevents water penetrating by means of capillary forces between the cable shoe and the underlying workpiece, and in addition prevents corrosion. Penetrating water can have a deleterious effect on the mechanical strength of the brazed joint. In addition this can impair the electrical properties of the connection. Since the brazing clip is larger than the cable shoe, a larger joint surface is formed, which in turn produces a smaller electrical contact resistance.
In railway signal and cathode protection systems that operate at low voltages and currents, it is particularly important to have a low overall contact resistance in the brazed joints in order to prevent interferences in the system. With large currents and voltages, a high contact resistance leads to the evolution of heat in the brazed joint, which may damage and/or melt the latter. Since the connection also has to withstand high return currents in the railway operations system it is important to have a low contact resistance in the brazed joint.
For a similar reason it is also important that the contact resistance is low in protective grounding.
A further advantage of this new brazing process is that the rail does not need to be grounded during the brazing. In the brazing process the electrode serves as one pole of the arc and the other pole is formed by the electrically conducting connecting piece, for example a cable shoe. In this case the electrically conducting connecting piece forms the negative pole, conventionally called the connection in grounding terminology. In the present invention the electrode may form the positive pole or negative pole, or alternately positive/negative pole. It is an advantage not to use the rail as a pole since secondary arcs may be formed between the cable shoe and rail, which may have a negative effect on the rail in the form of martensite formation. Moreover, by excluding the rail from the closed electrical circuit the cause of possible interfering signals in the rail and apparatus connected thereto is eliminated. The use of the cable shoe as one pole also eliminates a workstage and in certain situations grounding equipment associated with the brazing. The cable shoe can be connected to the electrical circuit via the guard ring in the brazing gun or via the cable connected to the cable shoe.
Another object of the present invention is that carbon powder from the carbon electrode is released during the brazing process and is deposited on the upper surface of the electrically conducting connecting piece, for example a cable shoe, and prevents the formation of a cavity in the cable shoe during the brazing procedure. In addition the electric arc is established between two carbon poles, which has a stabilising effect on the arc and counteracts the tendency of the current to fall over time. Furthermore the carbon has thermally insulating properties and exerts a temperature distributing function. The carbon powder from the carbon electrode consequently also acts as a buffer material, thereby preventing too high a temperature in, for example, the rail during the brazing process.
These and other objects are achieved by the method of the present invention for producing a new type of connecting piece of electrically conducting material, preferably a cable shoe, that is to be joined to another object of electrically conducting material by means of a temperature-controlled brazing process free from structural changes and martensite formation, characterised in that a tube of electrically conducting material and a brazing material suitable for hard brazing with an intermediate flux material suitable for soft brazing are pressed between an upper part of a tool and a lower part of a tool so as to form a connecting piece with a brazing clip pressed thereon and an intermediately located flux material, and in which the brazing clip consists of an upper part with two holes, a lower part and a rear part with two holes and the brazing clip is larger than a compact plate in one part of the connecting piece, and that the brazing clip is wholly of uniform thickness and that a bevelled section separates the lower part of the brazing clip from its rear part.